PROJ.4 has been placed under an MIT license. I believe this to be as close as
possible to public domain while satisfying those who say that a copyright
notice is required in some countries. The COPYING file read as follows:

All source, data files and other contents of the PROJ.4 package are available
under the following terms. Note that the PROJ 4.3 and earlier was “public
domain” as is common with US government work, but apparently this is not a well
defined legal term in many countries. I am placing everything under the
following MIT style license because I believe it is effectively the same as
public domain, allowing anyone to use the code as they wish, including making
proprietary derivatives.

Though I have put my own name as copyright holder, I don’t mean to imply I did
the work. Essentially all work was done by Gerald Evenden.